This invention relates generally to a system for detecting the passage or presence of an object. The invention finds particular application in the detection of vehicles and especially rolling stock or tracked vehicles. The invention will hereinafter be described with particular reference to the detection of tracked vehicles such as trains but it is to be understood that the scope of the invention is not limited in any way to this specific application.
Apparatus for detecting the passage of a train along a rail track must be capable of operating under widely ranging environmental conditions and of distinguishing between genuine signals and spurious signals which may arise from a variety of causes ranging from electrical noise originating from electric power lines, and thermal drift, to physical disturbances of the apparatus which may be caused for example by a workman's spade or pick.
In one prior art detection device a spaced transmitter and receiver are arranged so that a train's wheels pass between them and interfere with a transmitted electromagnetic signal. From the physical point of view the transmitter and receiver are exposed and are susceptible to mechanical damage whereas from the electrical point of view the system is affected by electrical noise and is not always suitable for the detection of trains moving at high speeds.
In another system known to the applicant amplitude variations in an electromagnetic field which are caused by the passage of a train are detected. This approach is highly susceptible to the influence of electrical noise.